As semiconductor processing progresses 45 nm node and beyond, the applications of ArF excimer lasers with increasing power and repetition rate require low loss, environmentally stable and laser durable coatings for optical components and systems for laser optics and precision optics. Surface and coating technologies will play a critical role in supporting the use precision optics and laser optics in DUV spectral regime. Wide band-gap fluoride thin films are generally preferred as coating for DUV uses.
At 193 nm wavelength, a well-prepared substrate surface is one of the dominant preconditions for good optical coatings, which includes surface finishing and cleaning prior to optical coatings, various methods such as optical polishing (Jue Wang, Robert L. Maier, John H. Burning, “Surface characterization of optically polished CaF2 crystal by quasi-Brewster angle technique,” SPIE 5188, 106-114(2003)); magnetorheological finishing (MRF) (Jue Wang, Robert L. Maier, “Quasi-Brewster angle technique for evaluation the quality of optical surface,” SPIE 5375, 1286-1294(2004)); diamond turning (Eric R. Marsh et al, “Predicting surface figure in diamond turned calcium fluoride using in-process force measurement,” J. Vac. Sci. Technol. B 23(1), p 84-89(2005)); Ultrasonic/megasonic and ultraviolet ozone cleaning (Jue Wang, Robert L. Maier, “Surface assessment of CaF2 DUV and VUV optical components by quasi-Brewster angle technique,” Applied Optics 45(22), 5621-5628(2006)); in-situ plasma ion cleaning (Jue Wang et al., “Color center formation of CaF2 (111) surface investigated by low-energy-plasma-ion,” Frontier in Optics, 88th OSA annual meeting (2004)). Improved optical surface quality extends component lifetime (U.S. Pat. No. 7,128,9847, “Improved surfacing of metal fluoride excimer optics” and U.S. Pat. No. 7,242,843, “Extended lifetime excimer laser optics”). Optical coating development has focused on fundamental understanding of film growth mechanism and plasma ion interaction (Jue Wang et al., “Correlation between mechanical stress and optical properties of SiO2/Ta2O5 multilayer UV NBF deposited by plasma ion-assisted deposition,” SPIE 5870, 58700E1-9(2005); Jue Wang et al., “Elastic and plastic relaxation of densified SiO2 films,” Applied Optics 47(13), C131-134(2008); Jue Wang et al, “Crystal phase transition of HfO2 films evaporated by plasma ion-assisted deposition,” Applied Optics 47(13), C189-192(2008); Jue Wang et al., “Wavefront control of SiO2-based ultraviolet narrow band pass filters prepared by plasma ion-assisted deposition”, Applied Optics Vol. 46(2), pp 175-179(2007); and Jue Wang et al., “Nanoporous structure of a GdF3 thin film evaluated by variable angle spectroscopic ellipsometry”, Applied Optics Vol. 46(16), 3221-3226(2007)). This understanding has led to new optical thin film design and coating process improvements; from oxide materials to fluoride materials and ultimately oxide-fluoride hybrids (U.S. Pat. No. 7,961,383, Jue Wang et al., “Extended lifetime of fluoride optics,” Boulder Damage Symposium, SPIE 6720-24 (2007); Jue Wang et al., “Structural comparison of GdF3 films grown on CaF2 (111) and SiO2 substrates,” Applied Optics Vol. 47(23), 4292 (2008); and Jue Wang et al., “Optical coatings with ultralow refractive index SiO2 films,” SPIE 7504, 75040F(2009)).
Wide band-gap fluoride thin films are generally preferred as coating for DUV uses. Using energetic deposition process is restricted for fluoride materials, because of fluorine depletion. The porous nature of thermal-evaporated fluoride coatings leads to measureable scatter loss and environmentally unstable. To overcome the porous nature of the metal fluoride coatings (MFx where x=2 or 3), hybrid oxide-fluoride coating were developed in which fluoride doped silica (F—SiO2) layers was inserted into stacks of MFx coating layers (U.S. Pat. No. 7,961,383). In addition, and outermost F—SiO2 layer was applied as a top of capping layer. However, these coating are relatively thick, and the problem still remains with regard to providing both environmental protection and anti-reflections properties to optics suitable for use in the DUV region. The present disclosure is directed to overcoming the shortcoming of the current environmental AR coatings for fluoride optics.